1. Field of the Invention
The present invention relates to pre-amplifier circuits in audio equipment.
2. Description of the Prior Art
Audio equipment having functions such as a recording function for recording on a recording medium a voice signal having a low signal level which is inputted from a microphone and a voice signal having a high signal level which is inputted from an external voice input terminal and a monitor function for reproducing and outputting the inputted voice signals has been known.
FIG. 3 illustrates a conventional pre-amplifier circuit provided in such audio equipment.
A pre-amplifier circuit comprises a voice input termina 101, a first switch 102, a buffer 103 an amplifier 104, a second switch 105, and a slide switch 106 for controlling both the switches 102 and 105. A voice signal having a low signal level from a microphone or a voice signal having a high signal level from an external voice input terminal is inputted to the voice input terminal 101.
When the voice signal having a high signal level from the external voice input terminal is inputted to the voice input terminal 101, the slide switch 106 is set at such a position that contacts 106b and 106c are connected to each other (a large signal-side position), and the first switch 102 and the second switch 105 are respectively connected to contacts 102a and 105a, as shown in FIG. 3. Consequently, the voice signal having a high signal level which has been inputted to the voice input terminal 101 is outputted through the buffer 103.
When the voice signal having a low signal level from the microphone is inputted to the voice input terminal 101, the slide switch 106 is set at such a position where the contact 106b and a contact 10a are connected to each other (a small signal-side position), and the first switch 102 and the second switch 105 are respectively connected to contacts 102b and 105b. Consequently, the voice signal having a low signal level which has been inputted to the voice input terminal 101 is amplified by the amplifier 104 and is outputted.
In the conventional pre-amplifier circuit, two signal paths, that is, a signal path including the buffer 103 and a signal path including the amplifier 104 are required. Further, in a case where the slide switch 106 is switched to the small signal-side position, when the voice signal having a high signal level is inputted, the amplifier 104 enters a hard gripped state. Accordingly, a power line greatly swings, so that noise is outputted.